A thermal recording sheet usually comprises a heat-sensitive coloring layer provided on a support, such as paper or film, and consisting mainly of a heat-sensitive color-developing composition. It is used for recording a color image when heated by a thermal head or pen, a laser, etc. The thermal recording system has a variety of advantages over other recording systems. For example, it is capable of quick recording, while requiring only a relatively simple apparatus, does not present any serious problem of noise or environmental pollution, and is inexpensive. Therefore, it is used for a wide range of applications, e.g. for facsimile devices, recorders, printers, ticket vending machines and label printers.
The recent improvement in the machines or apparatus with which the thermal recording sheets are used, and the development of new machines or apparatus have resulted in a demand for the correspondingly improved thermal recording sheets. For example, high image quality (dot reproducibility) and a uniformly glossy surface are required or preferred of the thermal recording sheets which are used with a CRT printer for producing a gradated image, or an instrumentation or label printer which is required to produce an image having a high contrast. On the other hand, a thermal recording sheet of high image quality and sensitivity having a dull surface is required for a facsimile device or an ordinary printer of the type which is principally used for producing a character image as it is of the prime importance that the characters which are reproduced are easy to read. Other recent improvements featuring all types of machines or apparatus under discussion have been a reduction in the power which is required for operating the machine or apparatus and a higher degree of resolution. These features have been calling for the development of a thermal recording sheet of appropriately improved sensitivity and image quality. Moreover, there has been a strong demand for a sheet having a uniformly glossy surface and a sheet having a uniformly dull surface which can be selectively used in accordance with any particular recording purpose.
It has hitherto been usual to manufacture a thermal recording sheet by coating a support, as of paper, with a heat-sensitive coloring layer, drying it and subjecting it to smoothing treatment, as by a supercalender, to improve its surface smoothness and thereby obtain improved image quality and sensitivity. It has, however, been difficult to obtain any satisfactory thermal recording sheet of high image quality and sensitivity having a uniformly glossy or dull surface.
Various methods have been proposed for producing a thermal recording sheet of high surface smoothness which achieves an improved contact with a thermal head and thereby improved image quality and sensitivity. They include a method which employs supercalendering to obtain a Bekk smoothness of 200 to 1000 seconds (Japanese Patent Publication No. 20142/1977), a method which provides a thermal recording layer on an undercoat layer containing wax and supercalendering it until it has a surface roughness R.sub.z not exceeding 2 .mu.m (Japanese Laid-Open Patent Specification No. 204594/1984), a method which employs supercalendering by hot metal rolls to obtain an optical surface roughness R.sub.p not exceeding 3.5 .mu.m (Japanese Laid-Open Patent Specification No. 237683/1986) and a method which comprises applying a thermal recording layer by a bent coating blade, drying it and smoothing it to a surface roughness R.sub.a not exceeding 1.2 .mu.m (Japanese Laid-Open Patent Specification No. 156086/1980). None of these methods has, however, been able to realize any surface smoothness providing satisfactory image quality or sensitivity. Moreover, there has not been available even any index of smoothness defining the level of image quality in a highly reliable way.
There have also been proposed various ways of producing a thermal recording sheet having an improved appearance. For example, Japanese Patent Publications Nos. 14531/1975 and 5947/1976 and Japanese Laid-Open Patent Specifications Nos. 46786/1981 and 64888/1985 propose certain recipes for the materials of a thermal recording layer which are intended for preventing it from being stained or having an uneven luster when it is calendered, and Japanese Laid-Open Patent Specification No. 155094/1984 proposes certain conditions for calendering. The unevenness of luster which is apparently due to the unevenness in formation of the support for a thermal recording sheet or the unevenness in coating of its thermal recording layer has been difficult to eliminate by any smoothing treatment, such as calendering. It has, therefore, been impossible to obtain any thermal recording sheet having a uniformly glossy surface. On the other hand, it has been usual to omit the smoothing treatment, such as calendering, or perform it only to a limited extent, in order to obtain a uniformly dull surface. It has, however, been possible to obtain only a thermal recording sheet of lower surface smoothness and therefore of low image quality and sensitivity.